This invention relates generally to radiation collimator devices and is concerned more particularly with an X-ray collimator having adjustable entrance shutter means which is sufficiently compact to operate within the recessed area of an X-ray generator port.
An X-ray generator usually comprises an oil-filled housing having therein an X-ray tube provided with an electron-emitting cathode and a spaced anode target. The cathode generally is disposed to direct a beam of electrons onto a small area, known as the "focal spot" area, of the anode with sufficient energy to generate X-rays in the target material. As a result, X-rays radiate from the focal spot area in all directions within the tube envelope. The useful portion of these X-rays pass, in the form of a conical beam, through an X-ray transparent window of a radially aligned port which is recessed in the wall of the housing. Thus, the focal spot area of the anode target ideally functions as a point source of the conical X-ray beam emanating from the port of the X-ray generator.
The X-ray beam may be directed, for example, through a selected region of a human patient for a limited interval of time, and impinge on an aligned surface area of a rectangular film. In this manner, the internal structure of the irradiated region may be reproduced on film for purposes of display and diagnostic study. However, in order to protect the patient from overexposure to X-radiation, it is required that the irradiated region of the patient be no larger than the effective area of the rectangular film. Accordingly, there may be mounted over the port of the X-ray generator a collimator housing having therein suitable X-ray absorbent shutter means for providing an adjustable exit aperture of the desired size. In this manner, the conical X-ray beam emanating from the port of the X-ray generator may be restricted by the collimator shutter means to the proper cross-sectional size and configuration for conforming to the effective area of the rectangular film.
The resulting image produced on the X-ray film by the collimated beam may not be sharply defined due to stray X-radiation generated in portions of the anode target outside of the focal spot area. In order to minimize the transmission of this off-focus radiation, the collimator may include an entrance aperture disposed as close as possible to the focal spot area of the anode target. Consequently, X-ray collimators of the prior art have been provided with entrance shutter means adapted to extend into the recessed port area of the X-ray generator. However, these prior art entrance shutter means generally operate inefficiently either by allowing an excessive amount of off-focus radiation to leak through, or by cutting off an excessive amount of useful X-radiation emanating from the focal spot area.
Therefore, it is advantageous and desirable to provide an X-ray collimator with adjustable entrance shutter means for efficiently limiting the passage of off-focus X-radiation while permitting the passage of X-radiation from an aligned X-ray source.